An electric storage device such as a lithium-ion battery may be overcharged or over-discharged due to variations in capacity between cells or a failure of a peripheral device such as a charger and a load. Therefore, a known battery pack may include an overcharge/over-discharge protection function. Such a battery pack may include an electronic switch such as an FET and a monitoring unit. The switch is connected in a current path in which a charging current or a discharging current flows. The monitoring unit is configured to measure a terminal voltage of the electric storage device. If the terminal voltage reaches a predetermined level during charging of the electric storage device, the switch is turned off to shut off the charging current so that the electric storage device is protected against overcharge.
A known battery pack including a protection function may have the following configuration. If a voltage between an input and an output of an electronic switch, that is, a voltage drop during charging is large, an abnormal condition in which an on resistance is excessively high due to a failure of the switch is determined. If the abnormal condition is determined, the switch is turned off to disable a battery.
In such a battery pack, the switch may not be turned off if a short circuit occurs between the input and the output of the switch or for some reasons. If the switch cannot be turned off, the protection function may not be properly performed. As a result, the electric storage device may be overcharged or over-discharged. Even if an abnormal condition of the switch is detected, the battery cannot be disabled because the switch cannot be turned off.
A turn-off problem of the switch may be detected in advance by turning the switch from on to off. However, power supply to a load may be stopped if switch failure detection is performed as such on a discharging current shut-off switch during discharging. If the switch failure detection is performed on a charging current shut-off switch during charging, power supply to the electric storage device may be stopped.